Gypsum wallboard is used in the construction of residential and commercial buildings to form interior walls and ceilings and also exterior walls in certain situations. Because it is relatively easy to install and requires minimal finishing, gypsum wallboard is the preferred material to be used for this purpose in constructing homes and offices.
Gypsum wallboard consists of a hardened gypsum-containing core surfaced with paper or other fibrous material suitable for receiving a coating such as paint. It is common to manufacture gypsum wallboard by placing an aqueous core slurry comprised predominantly of calcined gypsum between two sheets of paper thereby forming a sandwich structure. Various types of cover paper or similar functioning member are known in the art. The aqueous gypsum core slurry is required to set or harden by rehydration of the calcined gypsum, usually followed by heat treatment in a dryer to remove excess water. After the gypsum slurry has set (i.e., reacted with water present in the aqueous slurry) and dried, the formed sheet is cut into required sizes. Methods for the production of gypsum wallboard are well known in the art.
A conventional process for manufacturing the core composition of gypsum wallboard initially includes premixing dry ingredients in a high-speed, continuous mixing apparatus. The dry ingredients often include calcium sulfate hemihydrate (stucco), an accelerator, and an antidesiccant (e.g., starch). The dry ingredients are mixed together with a “wet” (aqueous) portion of the core composition in a mixer apparatus. The wet portion can include a first component that includes a mixture of water, paper pulp, and, optionally, one or more fluidity-increasing agents, and a set retarder. The paper pulp solution provides a major portion of the water that forms the gypsum slurry of the core composition. A second wet component can include a mixture of the aforementioned strengthening agent, foam, and other conventional additives, if desired. Together, the aforementioned dry and wet portions comprise the aqueous gypsum slurry that eventually forms a gypsum wallboard core.
The major ingredient of the gypsum wallboard core is calcium sulfate hemihydrate, commonly referred to as “calcined gypsum,” “stucco,” or “plaster of Paris.” Stucco has a number of desirable physical properties including, but not limited to, fire resistance, thermal and hydrometric dimensional stability, compressive strength, and neutral pH. Typically, stucco is prepared by drying, grinding, and calcining natural gypsum rock (i.e., calcium sulfate dihydrate). The drying step in the manufacture of stucco includes passing crude gypsum rock through a rotary kiln to remove any moisture present in the rock from rain or snow, for example. The dried rock then is ground to a desired fineness. The dried, fine-ground gypsum can be referred to as “land plaster” regardless of its intended use. The land plaster is used as feed for calcination processes for conversion to stucco.
The calcination (or dehydration) step in the manufacture of stucco is performed by heating the land plaster which yields calcium sulfate hemihydrate (stucco) and water vapor. This calcination process step is performed in a “calciner,” of which there are several types known by those of skill in the art. The calcining process itself is energy intensive. Several methods have been described for calcining gypsum using single and multi staged apparatus, as described in U.S. Pat. No. 5,954,497, which is incorporated by reference herein.
Calcined gypsum reacts directly with water and can “set” when mixed with water in the proper ratios.
During the conventional manufacture of gypsum board, gypsum slurry is deposited upon a moving paper (or fiberglass matt) substrate. The gypsum slurry may consist of several additives to reduce weight and add other properties. The moving paper or substrate itself is supported on a long moving belt. A second paper substrate is then applied on top of the slurry to constitute the second face of the gypsum board and the sandwich is passed through a forming station, which determines the width and thickness of the gypsum board. In such a continuous operation the gypsum slurry begins to set after passing through the forming station. When sufficient setting has occurred the board is cut into commercially acceptable lengths and then passed into a board dryer. Thereafter the board is trimmed if desired, taped, bundled, shipped, and stored prior to sale.
The majority of gypsum wallboard is sold in sheets that are four feet wide and eight feet long. The thicknesses of the sheets vary from one-quarter inch to one inch depending upon the particular grade and application, with a thickness of ½ inch or ⅝ inch being common. A variety of sheet sizes and thicknesses of gypsum wallboard are produced for various applications. Such boards are easy to use and can be conveniently scored and snapped to break them in relatively clean lines.
The process to manufacture gypsum wallboard is by some accounts over 100 years old. It was developed at a time when energy was plentiful and cheap, and greenhouse gas issues were unknown. This is an important consideration. While gypsum wallboard technology has improved over the years to include fire resistance as an attribute of certain wallboards, and gypsum wallboard testing has been standardized (such as in the American Society of Testing and Materials standardized test method C1396), there has been little change in the major manufacturing steps, and the majority of wallboard is still made from calcined gypsum.
As shown in FIG. 1, which depicts the major steps in a typical process to manufacture gypsum wallboard, gypsum wallboard requires significant energy to produce. The term “embodied energy” used herein may be defined as the total energy required to produce a product from the raw materials stage through delivery of finished product. As shown in FIG. 1, three of the illustrated steps (Step 102: drying gypsum, Step 103: calcining gypsum, Step 106: drying the boards) in the manufacture of gypsum wallboard require considerable energy. Thus the embodied energy of gypsum, and the resultant greenhouse gasses emitted from its manufacture, are very high. However few other building materials exist today to replace gypsum wallboard.
Energy is used throughout the gypsum process. After the gypsum rock is pulled from the ground it must be dried, typically in a rotary or flash dryer. Then it must be crushed and then calcined (though crushing often comes before drying). All of these processes require significant energy just to prepare the gypsum for use in the manufacturing process. After it has been calcined, it is then mixed typically with water to form a slurry which begins to set, after which the boards (cut from the set slurry) are dried in large board driers for about 40 to 60 minutes to evaporate the residual water, using significant energy. Often up to one pound (1 lb) per square foot of water needs to be dried back out of the gypsum board prior to packing. Thus, it would be highly desirable to reduce the total embodied energy of gypsum wallboard, thus reducing energy costs and greenhouse gasses.
Greenhouse gasses, particularly CO2, are produced from the burning of fossil fuels and also as a result of calcining certain materials. Thus the gypsum manufacturing process generates significant amounts of greenhouse gasses due to the requirements of the process.
According to the National Institute of Standards and Technology (NIST—US Department of Commerce), specifically NISTIR 6916, the manufacture of gypsum wallboard requires 8,196 BTU's per pound. With an average ⅝ inch thick gypsum board weighing approximately 75 pounds, this equates to over 600,000 BTU's per board total embodied energy. Other sources suggest that embodied energy is much less than 600,000 BTU's per board, and in a very recently built plant may be closer to 100,000 BTU per ⅝ inch thick board. Still, this is quite significant. It has been estimated that embodied energy constitutes over 30% of the cost of manufacture. As energy costs increase, and if carbon taxes are enacted, the cost of manufacturing wallboard from calcined gypsum will continue to go up directly with the cost of energy. Moreover, material producers carry the responsibility to find less-energy dependent alternatives for widely used products as part of a global initiative to combat climate change.
The use of energy in the manufacture of gypsum wallboard has been estimated to be 1% or more of all industrial energy usage (in BTU's) in the US. With 30 to 40 billion square feet of wallboard used each year in the US, some 200 trillion BTU's may be consumed in the manufacture of same. And as such, more than 25 billion pounds of greenhouse gasses are released into the atmosphere through the burning of fossil fuels to support the heat intensive processes, thus harming the environment and contributing to global climate change.
As mentioned above, the main ingredient in gypsum wallboard is calcium sulfate hemihydrate (calcium sulfate dehydrate before calcination), which on average is 90-99% of the wallboard core. Calcium sulfate dehydrate or raw gypsum rock is mined directly from the earth's crust, requiring mining, crushing and transportation energy. Additionally, increasing environmental concerns call for responsible manufacturing through use of any post-industrial waste in place of virgin resources such as mined gypsum rock. Recyclable by-products exist in many manufacturing industries such as coal-generated electricity, various metal purification processes, expanded perlite production and cement and lime kilns to name a few. An important aspect of lowering embodied energy is the incorporation of post-industrial waste into the wallboard core, which reduces energy usage associated with land-filling and disposing of these unwanted materials.
Prior efforts in gypsum manufacturing generally focused on reducing the weight of gypsum board, increasing its strength, or making minor reductions in energy use. For example, in U.S. Pat. No. 6,699,426, a method is described which uses additives in gypsum board to reduce the drying time and thus reduce energy usage at the drying stage. These attempts generally explicitly assume the use of calcined gypsum (either natural or synthetic), since gypsum wallboard manufacturers would find that redesigning the materials and mining procedures from scratch would potentially throw away billions of dollars of infrastructure and know-how, and render their gypsum mines worthless.
Therefore, given concerns about climate change, it would be desirable to manufacture wallboard which requires dramatically less energy usage during manufacturing. There is a need also for substantially reducing or eliminating energy intensive calcining and drying steps which are common to gypsum wallboard manufacturing.